Middle chainring chain noise

[tr85858]

I have a Shimano 105 triple on my Synapse road bike. I'm getting a lot of noise when the chain is on the middle front ring, regardless of which cog I'm in on the back. This disappears completely when the chain is on the largest front ring.
I've had the bike up on the stand and there is no rubbing against the front derailleur, or any other obvious problem. There isn't any skipping when i ride but it's always a relief to get onto the outer ring at the front. Any ideas what I have missed or is this just a something you have to put up with on a triple?
Thanks Tom

[binzer]

What type of noise is it? I.E. Rubbing, grinding, scraping, etc.? Also, does it make the noise when you have the bike on the repair stand or just under load? Look at the teeth of the center ring to check for excessive wear, worn teeth look like a shark's fin. Also check to see if the ring is bent. When you have it on the stand with the chain on the large or small ring spin the pedals and look a the crankset from the front or top to see if the center ring is bent or warped.

[DaveM]

Loose chainring bolts?